1. Field of the invention
The present invention relates generally to a subscription television system (pay-television system) and, more particularly, to a scrambling system of a television signal which may be received by any television receiver but can be reproduced properly only by the television receivers of the subscribers and can not be reproduced properly by the television receivers of non-subscribers.
2. Description of the Prior Art
In some subscription television system, it is desired that the television broadcasting service is charged to each subscriber not only by the channels, but by the programs. This can be accomplished by broadcasting a scrambled television signal which can not be properly reproduced by the normal television receiver, but can be properly reproduced by the television receiver of an authorized system subscriber by the employment of a suitable decoder.
According to the prior art scrambling system, the television signal is scrambled by merely changing the polarity of the video signal or by a minor change in the synchronizing pulses. Such simple scrambling methods, however, may be easily descrambled, or unscrambled, and therefore, the television programs can be easily reproduced by non-subscribers. On the contrary, if the television signal is scrambled in a complex manner, it becomes not only difficult to descramble the signal, but also may result in deterioration in the reproduced picture.